Thermally stabilized isotactic polypropylene fibers were developed using sulfonation procedure prior to carbonization stage. Sulfonation was performed to make the fibers infusible, intractable, and insoluble with concentrated sulfuric acid (approx. 95-97% conc.) at a temperature of 120 degrees C for sulfonation times ranging from 0.5 to 8 h. Sulfonation results in highly crosslinked structures which are thermally stabilized thus making the structure suitable to withstand the high temperatures involved in the carbonization stages. Structural characterization of sulfonated polypropylene fiber was carried out using combination of elemental analysis, differential scanning calorimetry, thermogravimetric analysis, and infra-red spectroscopy techniques with the aim of following the structural developments at a molecular level. Precursor fibers sulfonated for sulfonation times of 6 h or longer retained up to 46% of their mass at 1000 degrees C. DSC and IR-pectroscopy observations indicate the gradual loss of isotacticity and crystallinity with the progress of sulfonation treatment. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 234-245, 2012